These studies examine various problems involving sensitivity and specificity with special emphasis on vaccine approval. One study shows that imperfect sensitivity and specificity can seriously reduce the estimate of vaccine efficacy. Even a small decrement of specificity can reduce a highly efficacious vaccine to an apparently marginally efficacious one. A second study shows that a method proposed by a sponsor which gives a high sensitivity (choose the largest value as the cutoff for the test) leads to disastrously low specificity (and vice versa). A third investigation shows that the predictive values (positive or negative) are severely affected by the prevalence of the condition in question. This suggests that if the prevalence in the sponsor's sample is not representative of the prevalence that the product will be faced with in practice, the positive or negative predictive values may be far from the actual performance.